1. Field of the Invention
This invention relates to a non-linear voltage resistance element which is produced by molding and sintering metallic oxides and adapted for use in lightning arresters, surge absorbers, voltage stabilizing circuits and the like.
2. Description of the Prior Art
Included by the conventional type non-linear voltage resistance elements are:
A. zener diode utilizing `pn` junction of silicon, PA1 B. sintered silicon carbide body varister, PA1 C. selenium and copper oxides varister, PA1 D. sintered zinc oxide body varister, etc.
Used as criteria for evaluating the characteristics of the above enumerated non-linear voltage resistance elements are a non-linear coefficient .alpha. and a limiting voltage V.sub.T. The non-linear coefficient .alpha. is a value, when the volt-ampere characteristic is represented by the following formula: EQU I = KV.sup..alpha.,
wherein
K : constant
I : ampere
V.sub.T : voltage impressed on the element (limiting voltage), when the electric current becomes constant which is flowing through the element.
There arise the following problems with the aforesaid elements (a)-(d) from the criteria aforementioned:
The element (a) presents a great non-linear coefficient .alpha., while the limiting voltage V.sub.T is limited to from several to several tens of volts, and costly as compared with other elements.
The element (b) presents .alpha. of which value is as small as 2 to 3, and, in addition, may present further greater limiting voltage V.sub.T, but fails to present a reduced limiting voltage V.sub.T. The reason for this is that the thickness of an element can not be reduced up to below 0.5 mm, since the particle size of silicon carbide is as large as about 100 .mu..
The element (c) presents .alpha. of which value is as small as 2 to 3, and the limiting voltage V.sub.T thereof is high due to the use of a potential barrier at the contact portion between the metal and the semiconductor.
The element (d) presents .alpha. of which value is as large as 10 to 50, and the limiting voltage may be varied from 10 to 1,000 volts, owing to the particle size being as small as about 10 .mu.. However, the element has a disadvantage in that the non-linearity thereof is susceptible to deterioration due to the fact that zinc oxide, which is a principal component of the element, is chemically unstable.
Most promising from the above evaluation is a varister prepared from sintered zinc oxide (d). Generally, a sintered zinc oxide varister comprises aggregated particles of zinc oxide having relatively low electric resistance, around which each particle there is formed a thin particle boundary layer of a high electric resistance produced by the reaction of zinc oxide with added compounds. Thus, when the voltage is impressed on the varister, the aforesaid particle boundary layers no longer hinder the voltage with the result that the varister attains low conductivity. In referring to the structure described above, it is contemplated to provide a non-linear voltage resistor element by forming particle boundary layers of a high electric resistance around particles of a metallic oxide from which oxygen is harder to be free than from zinc oxide.